[AI assisted]: MM-62295: Search and index archived channels as well. (#29796)

We add a new field delete_at in the channels template.

This field is then searched in the SearchChannels function.

Also added tests to verify that archived channels are searched
properly, and also indexed correctly.

https://mattermost.atlassian.net/browse/MM-62295

```release-note
- Now archived channels are searchable with ES/OS if TeamSettings.ExperimentalViewArchivedChannels is enabled.
- If there are old channels which were archived before a bulk index was run, users would need to purge indexes, and do bulk index again. Because those old archived channels are removed from the index when a bulk index is run.
```


Co-authored-by: Mattermost Build <build@mattermost.com>

server/enterprise/elasticsearch/opensearch/opensearch.go

@@ -860,7 +860,7 @@ func (os *OpensearchInterfaceImpl) IndexChannel(rctx request.CTX, channel *model
 	return nil
 }
 
-func (os *OpensearchInterfaceImpl) SearchChannels(teamId, userID string, term string, isGuest bool) ([]string, *model.AppError) {
+func (os *OpensearchInterfaceImpl) SearchChannels(teamId, userID string, term string, isGuest, includeDeleted bool) ([]string, *model.AppError) {
 	os.mutex.RLock()
 	defer os.mutex.RUnlock()
 
@@ -925,6 +925,14 @@ func (os *OpensearchInterfaceImpl) SearchChannels(teamId, userID string, term st
 		})
 	}
 
+	if !includeDeleted {
+		query.Filter = append(query.Filter, types.Query{
+			Term: map[string]types.TermQuery{
+				"delete_at": {Value: 0},
+			},
+		})
+	}
+
 	buf, err := json.Marshal(search.Request{
 		Query: &types.Query{Bool: query},
 	})